The present invention relates to a gear pump which is adapted to convey a heated polymeric material without significant leakage. More particularly, the present invention relates to a gear pump of the type which is adapted to be positioned downstream of a plastic extruder so as to serve as a booster or metering pump. Such pumps typically comprise a pair of intermeshing gears, with the gears being rotatably supported by shafts which are mounted in journal bearings, and with one of the shafts extending through the housing and serving as the drive shaft for transmitting torque to the gears.
In the plastic industry, particularly in the man made fiber industry, pumps of the described type are employed as metering pumps which are positioned downstream of an extruder which heats and melts the high polymer plastic and supplies the resulting melt under pressures of up to about 100 bar and higher to the pump. The pump in turn increases the pressure of the high polymer melt to the required spinning or extrusion pressure, which is typically several hundred bars, and then delivers the melt to the spinning or extruding nozzle. When manufactured with adequate precision, the pumps operate reliably and provide a highly accurate flow rate.
It is recognized that a minimum play is unavoidable between the moving parts of gear pumps, and on the high pressure side of the pump, the melt is caused to be forced into the narrow gaps between the sides of the gears and the opposing side wall surfaces of the housing, starting from about the area in which the teeth initially engage. As a result, the melt tends to flow into the bearings, and to flow outwardly through the bearing of the drive shaft. This plastic may pass through the shaft seal and lead to undesirable contamination and deposits, which result in breakdowns.
In the case of gear pumps which serve to deliver lubricating fluids, it has previously been suggested that a secondary flow be established in addition to the main flow for the purpose of maintaining an adequate lubrication of the gear bearings. The secondary flow is guided through the individual bearings, and a steady flow rate of the lubricant is therefore maintained. It has also been proposed to use the change from high pressure to lower pressure which occurs in the fluid cells formed by the intermeshing teeth, so as to produce such a lubricant flow, note for example U.S. Pat. Nos. 3,447,472 and 3,490,382, and EPO Pat. No. 062,405.
EPO Pat. No. 062,405 relates to an improvement of the bearing lubrication of a gear pump. On each pump side, the roller bearings are separated from the supported gears by wearing plates. The journal bores provided in the wearing plates are kept sufficiently large so as to enable a flow of the lubricant through the existing play between the journal shafts and the walls of the journal bores. On its side facing the gears, each wearing plate has a radial recess, which is positioned in the area of the engagement of the teeth on the pump intake side. These recesses are respectively associated with only one gear and are directed to the axis of the same, so that the one recess of the driven gear is associated with the other recess of the follower gear. Both recesses extend respectively from the journal bore for the shaft in the wearing plate and lead up to the area between the root circle and the addendum circle of the respectively associated gear. Another recess is provided on each side of the gears opposite to the first mentioned recesses, but on the outside of the wearing plates, and these further recesses lead from the pump intake up to the associated journal bore for the shaft.
The increasing, enclosed volume of the fluid cells results in that the thereby developed underpressure sucks, via the two recesses located directly adjacent the gear, the lubricant out of the gear bearing and delivers it to the pump intake. A cross connection to the neighboring bearing located on the same side of the pump results in that the underpressure in the first bearing is also operative in the second bearing and generates a suction there as well. This suction is filled on both sides from the pump intake via the recesses respectively located on the same pump side as the first recesses, but provided on the back side of the wearing plate. The described operation is identical on both pump sides.
The measures described in the above prior art reference are not suitable for eliminating the initially described problems in the processing of plastic melts or the like. More particularly, the described generation of secondary flows serving the lubrication would not provide the required accuracy in the case of metering pumps used in the processing of spinning melts and in which the flow of the melt delivered by the pump must be accurately maintained within very narrow limits. Further, the guarantee of an adequate lubrication of bearings is not a problem with which the present invention is concerned.
It is accordingly an object of the present invention to provide a gear pump of the type which is adapted to precisely meter the required flow of a heated plastic melt, and which avoids leakage of the melt from the pump.
It is also an object of the present invention to provide a gear pump of the described type which effectively avoids the deposit of the melt in the bearings, and in particular the bearing of the drive shaft, and which provides a means for removing any such melt which may enter into the shaft bearing.